1. Field of the Invention
The present invention relates to a television receiver which can receive TV signals or FM broadcasting signals.
2. Description of the Related Art
The structure of a conventional television receiver is shown in FIG. 5. A mixer 31 is incorporated in an integrated circuit 30 and there are a tuning circuit, an amplifier and so on (not shown) outside of the circuit 30. A TV signal or an FM broadcasting signal which is selected by this tuning circuit is sent to the mixer 31. Using a local oscillation signal sent from a local oscillator (not shown) located in the integrated circuit 30, the mixer 31 converts the frequency of the TV signal into a frequency in the TV intermediate frequency band (54 MHz to 60 MHz in accordance with the Japanese channel specification), or the frequency of the FM broadcasting signal into a chrominance subcarrier frequency (C) in the intermediate frequency band. The frequency-converted TV signal or FM broadcasting signal is outputted in a balanced form through a balanced output terminal 30a or 30b of the integrated circuit 30 and sent to a balanced intermediate frequency tuning circuit 32.
The intermediate frequency tuning circuit 32 is made up of three (first, second, and third) tuning circuits 33, 34, 35 which are connected in parallel, and the second tuning circuit 34 and the third tuning circuit 35 can be connected with the first tuning circuit 33 through a first switch diode 36 and a second switch diode 37, respectively. Line voltage B is supplied to the mixer 31 through inductors 33a and 33b of the first tuning circuit 33. Line voltage B is also supplied to the anodes of the first switch diode 36 and the second switch diode 37 through the inductors 33a and 33b. When a TV signal is received, the first and second switch diodes 36 and 37 are turned off and only the first tuning circuit 33 in the intermediate frequency tuning circuit 32 is effective and tuned between a video intermediate frequency (P) and a chrominance subcarrier frequency (C) in the intermediate frequency band.
On the other hand, when an FM broadcasting signal is received, the first and second switch diodes 36 and 37 are turned on. The first tuning circuit 33 is connected in parallel to the second and third tuning circuits 34 and 35, which increases the Q factor for the tuning circuitry and causes tuning around the chrominance subcarrier frequency (C).
Next to the intermediate frequency tuning circuit 32 is a balanced input/unbalanced output intermediate frequency amplifying circuit 38 which is incorporated in the integrated circuit 30. The input ends of the intermediate frequency tuning circuit 32 and the intermediate frequency amplifying circuit 38 are connected to the balanced input terminals 30c and 30d of the integrated circuit 30. The unbalanced output end of the intermediate frequency amplifying circuit 38 is connected to an unbalanced output terminal 30e of the integrated circuit 30. Voltage is supplied from the intermediate frequency amplifying circuit 38 to this unbalanced output terminal 30e which is connected with a trap circuit 39. The trap circuit 39 has a first capacitor 39a, a first inductor 39b, and a second capacitor 39c which are connected in series from the unbalanced output terminal 30e to an intermediate frequency output end 40, in the order of mention. The first capacitor 39a is connected in parallel to a third switch diode 39d. Also, a third capacitor 39e is connected between the unbalanced output terminal 30e and IF output end 40. A series circuit from the point of connection between the first capacitor 39a and the first inductor 39b, which includes a varactor diode 39f and a second inductor 39g, is connected to the ground.
In addition, the cathode of a third switch diode 39d and the cathode of a varactor diode 39f are connected in series with the collector of a switch transistor 41 through the first inductor 39b and a resistor 39h. Line voltage is applied through a pull-up resistor 42 to the collector. The switch transistor 41 is also incorporated in the integrated circuit 30. The anode of the varactor diode 39f is biased by the resistors 39i and 39j. The cathodes of the first and second switch diodes 36 and 37 are also connected to the collector of the switch transistor 41.
In the above structure, when a TV signal is received, the switch transistor 41 is turned off. This turns off the first and second diodes 36 and 37 and causes the intermediate frequency tuning circuit 32 to operate as described above. Also, the third switch diode 39f is turned off. Therefore, the first capacitor 39a, first inductor 39b and second capacitor 39c which are serially interconnected, combined with the third capacitor 39e connected in parallel to all these, constitute a parallel resonant circuit whose resonance frequency is equal to the video intermediate frequency (−P) of the adjacent channel. Also, as line voltage is applied to the cathode of the varactor diode 39f, the series resonance frequency for the varactor diode 39f and the second inductor 39g is equal to the sound intermediate frequency (−S) of the adjacent channel.
Therefore, the overall transmission characteristic of the intermediate frequency tuning circuit 32 and the trap circuit 39 is as indicated by A in FIG. 6: namely attenuation occurs at the adjacent channel's video intermediate frequency (−P) and at the sound intermediate frequency (−S).
On the other hand, when an FM broadcasting signal is received, the switch transistor 41 is turned on. This turns on the first and second diodes 36 and 37 and causes the intermediate frequency tuning circuit 32 to operate in the same way as described above, except that, since the Q factor for the tuning circuit increases, there occurs a sharp change in the tuning characteristic, resulting in increased loss and signal level decline. Also the third switch diode 39f is turned on. Therefore, the first inductor 39b and second capacitor 39c which are serially interconnected, combined with the third capacitor 39e connected in parallel to all these, constitute a parallel resonant circuit whose resonance frequency goes down to the level of the video intermediate frequency (P). Also, the voltage of the cathode of the varactor diode 39f decreases, the series resonance frequency for the varactor diode 39f and the second inductor 39g is nearly equal to the middle point between the video intermediate frequency and chrominance subcarrier frequency ((C+P)/2).
Therefore, the overall transmission characteristic of the intermediate frequency tuning circuit 32 and the trap circuit 39 is as indicated by B in FIG. 6: namely attenuation occurs nearly at the middle point ((C+P)/2) between the video intermediate frequency and chrominance subcarrier frequency, and at the video intermediate frequency (P). However, the level at the tuning point C (chrominance subcarrier frequency) is lower than when a TV signal is received.
The conventional TV tuner intermediate frequency circuit as explained above uses three intermediate frequency tuning circuits and the trap circuit generates two trapping frequencies. For this reason, it has been necessary to use a larger number of components outside of the integrated circuit. As a result, it has been difficult to reduce the cost of the intermediate frequency circuit and decrease its size.